Apparatus for generating frequency modulated signals



D. C. ERDMAN Sept. 1, 1959 APPARATUS FOR GENERATING FREQUENCY MODULATED SIGNALS 2 Sheets-Sheet 1 Filed May l mmzm www,

2,902,658' 7 APPARATUS ORNGENRATING FREQUENCY y .MODULATED' SIGNALS Donald C. ErdmamPasadena, Calif., assigner to Electrol circults.-vlncorporated, Pasadena, Calif., a corporation L oi California Appliaauan May-1,19s1, serial No. 656,332

' ciaims. (ci. 332-2-16) This invention relates to` improved apparatus for generating frequency modulated signals for use in measuring the propagationtime' of Waves along a path. The invention.y has particular reference to use in producing ultrasomc waves such as are employed in the nondestructive testing of materials to detect hidden flaws.

In accordancewith the invention which is disclosed in copending application Serial No. 522,542, for Methods andfApparatus for Measuring Wave Propagation Time, the inventor and assignee of said copending application being thel samel as inthe instant application, improved apparatus is disclosed for measuring wave propagation time by the use of frequency modulated signals. In

accordance with the-invention whichv is, disclosed inthe aforesaidv copending lpatent application, two waves are produced intime sequence havingv frequencies which vary or, sweep periodically in substantially the same manner, with; the second wave starting at a later time and being oiset in frequency from that ofthe first wave. One or both ofvthewaves may be transmitted from the source toarreiiectingA target,` and the reflected waves are mixed -with one or` bothofthe wavesfwliich are then being produced-v by the source..4 An indicator arrangement provides an. indication ofthe occurrence of heterodyne sigvnalshavin'g a certain frequency.

The 'present inventionrelates to an improved arrange'- ,m'en't for generating. two Wavesin time `sequence having vfrequencies vwhich vary ors'weep periodically in substantially'tlie same manner; The" present invention discloses a greatly' simplifiedv arrangement for producing such Waves.'

In accordance with; the present invention, a transistor oscillator circuit' is arrangedinv the form of a negative resistance oscillator A signal of sawltootli wave` form isapplieditoone* electrode of the' transistor oscillator to cause the frequency of oscillation of theA circuit to sweep ,through a predetermined frequencyran'ge in response to jea'cl'i"'ratcursiorr of the: saw-toothl signal.. A signal of "square waveV formisappiied to another electrode of the 4'transistorand it' Iimitsthe' current which-flows through 'tlietransistorin accordance with the square wave form ofitli'esignal so'astocause the frequency of-'os'cillation-to change'om onelevel toanother level'in` accordance with 'Y ejach cli'ange in level ofthe signal of square wave form. :The signal of` saw-tooth/wave form andthe signal* of square' wave formarepcaused to occur in synchronism.

'Quoto tlieactionofithe signal'ofsquare waveform, the rfrequency"ofv oscillations sweeps through two frequency ranges, with the lowest frequency 'range occurring during wave.

In some instances the sweep infrequency of the two Patentedrv Sept. 1.,. 1.959:V

waves is not identical, and it is desirable to correct for any differences in the two sweeps. This may be done in accordance with they present invention by changing the amplitudes of the successive saw-tooth signals inaccordance with the square wave signal. This may be done by providing a simple coupling between the source whichV produces the squareY wave signals andthe sweep generator whichrproduces thesaw-tooth signals.

One of the advantages of the invention is to provide a n ovel oscillator circuit which may be employed in non destructive testing of materials. y n.

Another advantage of the present invention is to pro vide a novel improved oscillator arrangement for generating the frequency modulated waves employed in ultrasonic apparatus. y v

Another advantage of the invention is the provision of an oscillator circuit having a correction arrangement for correcting differences or non-linearities in the frequency modulated signals which are produced by the oscillator circuit and usable in ultrasonic apparatus.

The present invention contemplates apparatus for measuring wave propagation time by using frequency modulated signals wherein tw'o'waves produced in time sequence varyV periodically, with one wave starting at' a later time and being oiset in frequency from the other". One or' more of the waves are' transmitted to a reflecting targetV and the reectedwave is mixed with one" or more of the waves which are then produced bythe' source `and present an indication of target information on ran indicator' represented by the occurrence of heterodyne'signals having a certain frequency. The improved oscillator arrangement includes transistors and tuning networks for attainingthe desired results. *y

The foregoing and other objects and advantages of the invention will appear moreV fully hereinafter from a: corisideration of the detailed description which follows, taken together with the accompanying sheets of drawing wherein several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and descriptionL only an'd are notintended as a detinition ofthe limits of theinvention.

In the drawings:

Fig. 1 is a circuit diagram partially in block forman'd partially in schematic form, showing the presentinventionemployed inultr'as'onic apparatus.

Fig. 2 illustrates the wave forms of various signals which are' produced in the apparatus of Fig. l. y

Referring tothe drawings, and more particularly to Fig'. 1, there is' showna time pulse generator 12, whichprovides a series of equally spacedpulses A (see wave forms in Fig.' 2)V which control the sequenceof operations' in thevr apparatus; The timing. pulseV generator is a variable repetition rate type and the rate or' periodicity of the pulses is controlledby the voltage which isproduced by variable Voltage source 14. The variable voltage source 14' may comprise apotentiometer 16 which is connected across a! source of-poteutial- 1'8 and which is driven by am'otor 20, or it may be a conventional electronic' sweep generator which provides periodic voltage excursions.V`

Thetiming pulses-1 A are applied to a sweep generator 22 and toa-bistable circuit Z4. The sweep' generator produces voltage excursions B (see Fig. 2) having sawtooth wave form insyfnchronism with the timing pulses A. The bi-stable circuitproduces signals C- (see Fig, 21) of rectangular/waveV form in synchronism with the voltage excursionslB;

A'IfhesfsaW-tooth: sign-als 1?`v are applied toA a circuit arrangement inclndingi` ay potentiometer 106` connected `of the cathode-ray tube.

power amplifier 66 which is a variable. gain soy that a,V

signal having the lgeneral wave form of F (Fig. 2) is connected to a crystal or other transducer 44, which serves to transmit and receive ultrasonic signals, with said crystal being connected to ground in the conventional manner as shown by symbol. As illustrated in the wave form F of Fig. 2, the amplitude of the signal which is conveyed through the power amplifier 66 is reduced during alternate cycles of operation, and the wave of reduced amplitude is mixed with the echo signals in the amplifier 52 to provide heterodyne signals.

ln conventional practice, the transducer has a portion thereof immersed in liquid which serves to convey the ultrasonic waves with much greaterrefiiciency than would be possible in the air. The output of the power amplifier 66 is fed into transducer 4141v and simultaneously into 'the amplifier 52. The echo signals which vare received by the transducer 44 are also fed into the amplifier 52. VThe output of the amplifier 52 has a wave form G (see Fig. 2) which is fed through a narrow band filter 56, thence through a detector 5S, and a video amplifier 60.

The narrow band filter 56 may be arranged to pass heterodyne signals having any desired frequency. By way of example, the filter 56 may be tuned to pass signals having la frequency of 455 kilocycles. Such filters are readily available because they are conventional types.

The detected signal is amplified by a Video amplifier 60 and applied to one set of the deflection plates of a conventional cathode-ray tube represented as an indicator 62. The variable voltage which is produced by the source 14 is applied to the other set of the deflection plates of the cathode-ray tube or indicator 62. Such a deection arrangement provides an A-type scan so that the location of the detected signal along the horizontal axis of the cathode-ray tube provides a measure of the distance to the target.

The distance to the target is also indicated by the time between the pulses which are produced by the timing pulse generator at the instant when a signal appears on the screen of the cathode-ray tube. lf desired, the signal which is produced by the variable voltage source 14 may be adjusted manually and the distance to the target may be ascertained by observing the manual setting of the voltage source when a signal is produced on the screen However, it is preferable that the signal which is produced by the source 14 be varied cyclically, as indicated in Fig. l, so that plural reflecting surfaces may be indicated during each cycle of operation.

The oscillator circuit comprises a transistor 100 and a resonant circuit 102 which are arranged in the form of a negative resistance oscillator. The transistor 100 should be a type having an alpha characteristic which is greater than l in order to produce the desired negative resistance characteristic. A condenser 104 is connected between the collector electrode and the grounded terminal of the resonant circuit 102, and this condenser provides a very low impedance path for the radio frequency energy so that the resonant circuit 102 is effectively coupled between the base and the collector electrodes of the transistor.

The sweep signals B which are produced by the sweep generator are applied through a potentiometer 106 and a wave shape network S and a coupling condenser 110 to the base electrode of a transistor 112. The transistor 112 serves as an emitter follower circuit, and it serves as a low impedance circuit for controlling the signal which is applied to the emitter electrode of the oscillator transistor 100.

The saw-tooth signals B are applied through the low impedance circuit of the transistor 112 and a radio frequency coil 114 to the emitter electrode of the transistor 100. The saw-tooth signals cause the frequency of oscillation of the oscillator circuit to sweep through a predetermined frequency range in response to each excursion of the saw-tooth signal.

A transistor 116 serves as a low impedancelcircuit for limiting the current which flows through the collector electrode of the oscillator transistor 100. The transistor 116 has its base electrode coupled through a coupling condenser 11S to receive the signals C of square wave form which are produced by the bi-stable circuit. Thus, the current which flows through the emitter circuit of the transistor 10i) is limited inaccordance with the wave form of vthe square wave signals C, and this causesthe frequency of oscillation of the loscillator to change from one level to another level in accordance with eachl change in level of the signal of square wave form.

These changes in level are synchronized with the sawtooth signals so that the oscillator repeatedly produces two waves in time sequence having frequenciesjwhich sweep periodically in substantially the same manner, with the second wave being offset in frequency from that of the first wave.

Any differences in the two sweeps in frequencyrange may be corrected by a signal applied through the lead 12d to the sweep generator. The lead 120 is connected to receive the square wave signals C which are produced by the bi-stable circuit, and these signals are applied to the sweep generator to alter the charging current'of a condenser 122 in the sweep generator in accordance'with the wave form of the square wave signal.

The sweep generator comprises a switching transistor 124 and a follower transistor 126. The switching transistor 124 controls the charging and discharging of the condenser 122 from the source of-potential 128 through the resistor 130 so as to produce signals of saw-tooth wave form.

Every other saw-tooth signal may be altered by the square wave signals C which are applied through'the lead and a potentiometer 132 to the condenser 122.

The resulting saw-tooth signals are applied through the follower transistor 126 to provide the output signals B from the final transistor of the sweep generator.

From the foregoing it will be seen thatv the present invention embraces two species of an improved oscillator arrangement for generating the frequency vmodulated Waves employed in the ultrasonic apparatus. The first species of the invention relates to the oscillator circuit itself. The second species concerns the oscillator circuit plus a correction arrangement for correcting f or any differences or non-linearities of the frequency modulated signals which are produced by the oscillator circuit..

While several embodiments of the invention have been illustrated and described in detail, it is to be expressly understood that invention is not limited thereto. Various changes can be made in the design and-arrangement of the parts without departing from the spirit and scope of the invention as will now be understood by. those skilled in the art. v

1. In an apparatus for generating frequency modulated signals for measuring the propagation time `of waves travelling along a preselected path, in combination, first generating means for generating a first signal having a first wave form; second generating means for-generating a second signal having a second wave form; oscillator means for producing an oscillatory signal v'and-haying respectively a first and second-inputfcircuitandan output circuit, said first and second input circuits being connected respectively to said first and second generating means so that said first and second'signals are respectively applied thereto and mixed together with said oscillatory signal to modulate the same and produce a first series of alternate modulated wave trains, one of said wave trains being modulated in accordance with a first preselected combination ofv said rst and second signals and the other of said wave trains in said series being modulated in accordance Witha second preselected combination of said first and second signals; and mixing means having a rst input connected to said output circuit of said oscillator means for receiving said series of alternate wave trains and having a second input connected to said second generating means for receiving said second signal thereof, said mixing means mixing the signals applied thereto to produce a second series of yalternate modulated wave trains wherein the amplitude of one of said second series of wave trains is larger than the amplitude of the other of said second series of wave trains.

2. Apparatus as claimed in claim l wherein said iirst and second generating means are synchronized by synchronizing means so that said `frst and second signals have a pre-selected phase relationship therebetween.

3. In an apparatus for generating frequency modulated signals for measuring the propagation time of waves travelling along a preselected path, in combination, first generating means for generating a sawtooth signal; second generating Imeans for generating a square wave signal; oscillator means for producing an oscillatory signal and having respectively a first and second input circuit and an output circuit, `said tirst and second input circuits being connected respectively to said rst and second generating means so that said sawtooth and square wave signals are respectively applied thereto and mixed together with said oscillatory signal to modulate the same and produce a rst series of alternate modulated wave trains, one of said wave trains being modulated in accordance with a rst preselected combination of said sawtooth and square wave signals and the other of said wave trains in said series being modulated in accordance with a second preselected combination of said sawtooth and square wave signals; and mixing means having a rst input connected to said output circuit of said oscillator means for receiving said series of alternate wave trains and having a second input connected to said second generating means for receiving said square wave signal thereof, said mixing means mixing the signals applied thereto to produce a second series of alternate modulated wave trains wherein the amplitude of one of said second series of wave trains is larger than the amplitude of the other of said second series of Wave trains.

4. In an apparatus for generating frequency modulated signals for measuring the propagation time of waves travelling along a preselected path, in combination, first generating means for generating a first signal having a first wave form; second generating means for generating a second signal having a second wave form; oscillator means for producing an oscillatory signal and having respectively a rst and second input circuit and an output circuit, said irst and second input circuits being connected respectively to said first and second generating means so that said rst and second signals are re- 5pt-@tively applied thereto and mixed together with said oscillatory signal to modulate the same and produce a first series of alternate modulated wave trains, one of said wave trains being modulated in accordance with the sum of said rst and second signals and the other of said Wave trains in said series being modulated in accordance with the difference between said rst and second signals; and mixing means having a iirst input connected to said output circuit of said oscillator means for receiving said series of/alternate wave trains and' having a second input connected to said second generating means for receivin-g said second signal thereof, said mix-ing means mixing the `signals applied thereto to produce a second series of alternate modulated wave trains wherein the amplitude of one of said second series of wave trains is larger than the amplitude of the other of said second series of wave trains.

5. In an apparatus for generating frequency modulated signals for measuring the propagation time of waves travelling along a preselected path, in combination, first generating means for generating a sawtooth signal', second generating means for generating a square wave signal; oscillator means for producing an oscillatory signal and having respectively a iirst and second input circuit and an output circuit, said rst and second input circuits being connected respectively to said il'st and second generating means so ythat said sawtooth and square Wave signals are respectively applied thereto and mixed together with said oscillatory signal to modulate the same and produce a rst series of alternate modulated Wave trains, one of said wave trains being modulated in accordance with the sum of said sawtooth and square wave signals and the other of sai-d wave trains in said series being modulated in accordance with the difference between said sawtooth and square wave signals; and mixing means having a first input connected to said output circuit of said oscillator means for receiving said series of alternate wave trains and having a second input connected to said second generating means for receiving said square Wave signal thereof, said mixing means mixing the signals applied thereto to produce a second series of alternate modulated wave trains wherein the amplitude of one of said second series of wave trains is larger than the amplitude of the other of said second series of Wave trains.

6. Apparat-us -as claimed in claim 5 'wherein said iirst and second generating means are synchronized by synchronizing means so that said irst and second signals have a preselected phase relationship therebetween.

References Cited in the le of this patent UNITED STATES PATENTS 

